2,4,6-Tris(2,4,6-tribromophenoxy)-1,3,5-triazine has been used as a flame retardant for plastic products. This compound may be synthesized by reacting cyanuric chloride with an alkali metal phenolate, typically sodium phenolate. During the reaction, an alkali metal halide, typically sodium chloride is formed as a by-product and thus the resulting product may be contaminated with a halide ion source. The product contaminated with the halide-ion source may result in various shortcomings when used as a flame retardant for flammable plastic materials. When thermoplastic materials are rendered flame retarded it is conventional to blend the thermoplastic material and the flame retardant in molten state and the molten blend is molded into shaped articles. The halide ions from their source contained in the flame retardant may promote corrosion of metallic parts of extruders or other mixing apparatus as well as molds and other metallic parts of shaping machines. The halide ions per se and rust pieces resulting from the corrosion of mixing or shaping machine parts may adversely affect the performance of shaped articles including heat resistance, electrical properties and tracking resistance in particular. Flame retardants are blended with thermosetting plastic materials for rendering them flame retarded. In this case, a dispersion or solution of solid flame retardant powder is added, for instance, to a resin varnish. Then prepregs are prepared from the varnish for fabricating a metal clad laminate for the production of a printed circuit board. If the flame retardant contains a halide ion source at unacceptable level, the electrical properties of the board is greatly compromised.
2,4,6-Tris(2,4,6-tribromophenoxy)-1,3,5-triazine (hereinafter simply called “brominated phenoxytriazine”) was first described in French Patent No. 1,566,675 wherein the compound is synthesized by adding a suspension of cyanuric chloride in acetone to a solution of sodium 2,4,6-tribromophenolate in benzene/acetone mixture.
U.S. Pat. No. 3,843,650 discloses the synthesis of brominated phenoxytriazine by the addition of cyanuric chloride to an ethanolic solution of sodium tribomophenolate. U.S. Pat. No. 5,965,731 discloses the synthesis of brominated phenoxytriazine by successively adding an aqueous solution of sodium hydroxide and a solution of cyanuric chloride in acetone to a solution of tribormophenol in acetone.
U.S. Pat. No. 4,039,538 discloses a reaction of an alkali metal tribromophenolate dissolved in an alkylene glycol monoalkyl ether such as methyl- or ethylcellosolve with cyanuric chloride.
JP 7/25859A, JP 7/25860A and JP 7/25861A disclose a reaction between a concentrated aqueous tribromophenolate solution/methylene chloride mixture and a solution or suspension of cyanuric chloride in methylene chloride in the presence of a tertiary amine and/or phase transfer catalyst.
Whichever process is employed, the concentration of alkali metal halide such as NaCl or KCl in the reaction system increases as the reaction proceeds. At the same time, brominated phenoxytriazine formed will crystallize out because its solubility in water and organic solvents is very low. Therefore, it is inevitable to entrain the halide salt by brominated phenoxytriazine when crystallizing from the reaction mixture. In order to minimize the entrainment of the halide salt, it would be effective to maintain the concentration of the halide salt in the reaction system as low as possible by decreasing the amounts of charged alkali metal phenolate and cyanuric chloride per unit volume of the reaction medium. This approach naturally decreases the amount of the target product to be recovered as crystals and is, therefore, hardly applicable to an industrial scale production due to low productivity.
Once contained in the crystals, it is difficult to remove the halide salt from brominated phenoxytriazine by washing with a solvent of the halide salt such as water, ethanol or acetone. Accordingly, one of other conceivable approaches to reduce the halide salt content would be dissolving the crystals containing the halide salt in a large volume of an organic solvent in which the crystals are soluble and recrystalling the product from the solution, or reverse extracting the halide salt from the above solution and then evaporating the organic solvent. It is also conceivable to repeat the washing of finely divided crystals containing the halide salt with water or ethanol. However, all of these approaches are hardly applicable to an industrial scale production due to low productivity.
Accordingly, a need exists for a flame retardant composition comprising 2,4,6-tris(2,4,6-tribromophenoxy)-1,3,5-triazine which has a minimum content of halide salts and other deteriorative impurities. Also, a need exists for a process for producing said flame retardant composition.